Vegetation cutting devices such as string trimmers are known power operated tools for cutting grass and other foliage and vegetation. Typically string trimmers are powered by petrol or electricity and an engine or motor drives a cutting head. The cutting head can comprise one or more rotating blades, in which case the trimmer is generally known as a brush cutter or one or more rotating cutting lines, in which case it is known as a string trimmer.
A vegetation cutting device such as a string trimmer comprises a rotating cutting head with a cutting line mounted thereto. Rotation of the cutting head causes the cutting line to rotate which defines a cutting swath. The cutting line rotates and the leading edge of the cutting line impacts and cuts vegetation. During use of a string trimmer the speed of the cutting line can vary depending on how the string trimmer is used. For example when the cutting line is use with heavy vegetation, the cutting head can slow down or even stall. When the cutting head rotates at a slower speed, the cutting line will not cut the vegetation.
It is known to control the speed on vegetation cutting devices. For example EP2499895 discloses a lawn mower which includes a speed control device which can maintain a constant speed control. The lawn mower comprises a speed sensor which is mounted in the housing and the speed sensor detects the speed of the blade. The speed of the blade is detected with magnets mounted on the blade. A problem with the speed sensor is that it is dependent on a precise positional relationship with the blade. If the blade hits a rock, the blade can be deformed or the magnet can be dislodged and the speed sensor may not be able to detect the rotation of the blade. Furthermore the magnet on the blade has to be replaced every time the blade is replaced.
DE202011000540 discloses a brush cutter with a speed sensor and a speed control system. The speed sensor comprises a hall sensor and a magnet mounted on the motor. A problem with this brush cutter is that the cutting head rotates at the same speed as the motor. This means that the motor is inclined to stall when the cutting head is used in heavy vegetation and the speed of the cutting head is reduced. In order to maintain a constant speed, the speed control system has to adapt to large changes in the speed of the motor. This means that significant energy is required to maintain the cutting head at the constant cutting speed.
Embodiments of the present invention aim to address the aforementioned problems.